In some ultrasound systems used for forming images of the interior of a patient's body, pulses of ultrasonic waves are directed into the body from a transducer held against it and reflection of these waves from structures within the body causes the transducer to produce corresponding electrical waves that are used to form the desired image. For various reasons, structures near the surface of the body and therefore near the transducer are not sharply focussed so that when such structures are of interest, it has been customary to place them within an area of better focus by inserting a stand-off device between the transducer and the patient's body. This also increases the field of view around these structures so that they can be more easily identified. In order to provide good acoustic coupling, stand-off devices have been comprised of a liquid-filled chamber having a diaphragm portion at one end, which is placed in intimate contact with the body, and a membrane portion at the other end, which is placed in intimate contact with the transducer. In some cases, no membrane is used and the transducer is placed in intimate contact with the liquid in the chamber.
A major problem with stand-off devices is that the quality of the image at one or more ranges is impaired by the low amplitude reflections of the ultrasonic waves of the pulses at the liquid/diaphragm and diaphragm/body interfaces. These reflections arrive at the transducer at about the same time as waves reflected from body structures that are close to the diaphragm, thereby deteriorating their image, and yet the attainment of a better image of these structures was the reason for using the stand-off device. It is also possible that the low amplitude waves reflected from the interfaces will be reflected by the transducer and again reflected at the liquid/diaphragm and diaphragm/body interfaces so as to reduce the quality of images of structures located in the body a distance from the diaphragm that is the same as the distance between the diaphragm and the transducer.